The Global Positioning System (GPS) consists of a constellation of orbiting satellites that transmit timing information and the satellite's ephemerides via microwave radio. Position determination devices determine position by analyzing signals received from four or more satellites. Any of a number of known methods can be used to determine position.
One frequently used method for determining position calculates pseudoranges that are then used to determine position. Pseudoranges are calculated by measuring the time it takes for the signal to travel from the satellite to the receiver. The satellites mark their transmissions digitally and the receiver compares the time it receives the time mark with its own time clock. The time delay, referred to as transit time, is typically in the range of about 70-90 milliseconds. Distance to each satellite (pseudorange) is then determined by multiplying transit time of each received signal by the speed of radio transmissions (approximately 300,000,000 meters/second).
Signals from each GPS satellite include the satellites ephemeris. The ephemeris indicates the location of each satellite. The position of the position determination device is then determined by a geometric calculation that uses the known satellite positions and calculated distances (pseudoranges). GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axes. These positions are often transformed into latitude, longitude, and height relative to the WGS84 ellipsoid.
One factor that introduces error into the process of determining location is atmospheric conditions. Another source of error results from the intentional introduction of error into the transmitted ephemerides and clock by the U.S. Air Force (referred to hereinafter as "selective availability" or "S/A"). The GPS navigation signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is currently specified by the Department of Defense (DOD) to be within 100 meters horizontal position 95 percent of the time and 300 meters 99.99 percent of the time. Errors also result from atmospheric conditions. Though the specified horizontal accuracy may be adequate for some applications such as navigation of a vessel in the open ocean, other applications require an increased level of accuracy.
One method for obtaining a more accurate determination of position is known as Differential GPS (DGPS). DGPS systems receive correction data broadcast from a DGPS reference station. DGPS reference stations are located at fixed and known locations and each DGPS reference station transmits correction data. By using receiver correction data along with signals received directly from GPS satellites, DGPS systems can accurately determine position. DGPS systems typically determine position in one of two ways. Traditionally, positions have been calculated using code phase differential techniques. These are normally referred to as DGPS. More recently, carrier phase techniques have been used to determine position. These systems are referred to as Real Time Kinematic (RTK) systems.
DGPS reference stations may be dedicated facilities with permanent and/or extensive broadcast capabilities or may be simply a transient DGPS receiver with data transmitter located at a known location. DGPS reference stations transmit either their calculated corrections to the GPS signals or their actual observations of the GPS signals (raw data), or both. When transmitting calculated corrections, errors due to atmospheric (troposphere, ionosphere, etc.) and errors due to satellite timing/clock (both intentional and process noise) are represented by the correction value. The application of these corrections at a DGPS receiver will compensate for these error sources.
Differential GPS reference stations may also transmit their observations of the GPS signals for each satellite. This method of transmission is popular with RTK positioning techniques and systems due to the nature of typical RTK processing methods. When using this type of data format, errors associated with atmospherics and satellite timing/clock errors are removed at the moving/roving/differential GPS receiver. Most manufacturers of RTK systems typically broadcast this data in a format unique to the particular manufacturer.
Many of the GPS reference stations broadcast in a format that conforms to standards established by the Radio Technical Commission for Maritime services (RTCM). These standards specify format, communication bands, and messages for a differential correction GPS service. Correction data that conforms to the RTCM format is broadcast by the US Coast Guard and others to assist in maritime navigation. The US Coast Guard has regional DGPS reference stations that calculate and broadcast correction data using the RTCM format. The RTCM correction data broadcast by some US Coast Guard DGPS reference stations includes carrier phase observable data while data broadcast by other facilities only includes code phase correction data. Other agencies and port authorities throughout the world broadcast differential correction signals conforming to the RTCM format for navigation in and around coastal areas. Both raw observable data and RTCM "correction data" are referred to hereinafter as "correction data" since both forms of data allow for correction to be made to position.
FIG. 1 shows a prior art position determination system 10 for determining position using correction data originating from a DGPS Reference Station that transmits in a RTCM format. Position determination system 10 is shown to include housing 17 that contains beacon antenna 11 and beacon receiver 13. Housing 18 is shown to include GPS antenna 12 and GPS receiver 14. Both housing 17 and housing 18 are coupled to a third housing that contains DGPS processor 19 by electrical cable. Battery 15 is connected by electrical cable to DGPS processor 19 for providing electrical power to the components of position determination system 10. Data logger 16 is also shown to be coupled via electrical cable to DGPS processor 19. Data logger 16 typically includes a display and function keys so as to allow users to view output and to input data as required for the operation of position determination system 10. In operation, beacon antenna 11 receives differential correction signals from a Reference Station that broadcasts in a RTCM format and couples the signals to beacon receiver 13. Beacon receiver 13 demodulates the RTCM signals so as to obtain correction data that is then coupled to DGPS processor 19. GPS antenna 12 receives signals from satellites of the GPS and couples the signals to GPS receiver 14. GPS receiver 14 demodulates the signals from GPS satellites and processes the incoming data, which is then coupled via electrical cable to DGPS processor 19. DGPS processor 19 then uses the data from beacon receiver 13 and GPS receiver 14 to accurately determine position.
One proposed new system for correcting position determination signals from satellites is the Wide Area Augmentation System (WAAS). The WAAS is designed for use with aircraft operations. The WAAS is designed to provide a system that has sufficient integrity such that position may be determined with sufficient reliability and accuracy for aircraft operations. The WAAS includes satellites for transmitting signals and a ground network that augments GPS such that GPS may be used as a primary navigation sensor for aircraft. The WAAS augments GPS with a ranging function, (which improves availability and reliability), differential GPS corrections (which improves accuracy), and integrity monitoring (which improve safety).
Prior Art FIG. 2 shows a proposed WAAS that includes WAAS satellite 4 that broadcasts GPS integrity and correction data, and a ranging signal that augments GPS. The WAAS ranging signal is GPS-like and may be received by slightly modified GPS receivers. More specifically, the WAAS signal will be at the GPS L1 frequency and will be modulated with a spread spectrum code from the same family as the GPS C/A codes. The code phase and carrier frequency of the signal is controlled so that the WAAS satellite will provide additional range measurements to a GPS user. The WAAS signal will also carry data that contains differential corrections and integrity information for all GPS satellites, as well as for the geostationary WAAS satellite 4.
The ground network shown in FIG. 2 accumulates differential corrections and integrity data at wide area Reference Stations (WRS) 2 that are widely dispersed. WRS 2 process the raw data received from GPS satellites to determine integrity, differential corrections, residual errors, and ionospheric delay information for each monitored satellite. They also develop ephemeris and clock information for the WAAS geostationary satellite 4. All of this data is accumulated at Wide area Master Site (WMS) 3 and is packaged into the WAAS message that is uplinked to the WAAS geostationary satellite 4 that broadcasts the WAAS signal. Aircraft such as aircraft 5 receive signals from GPS satellites such as GPS satellite 1 and receive the WAAS signal that then allows for accurately determining the position of aircraft 5. The WAAS signal does not interfere with GPS signals because the received WAAS signal has approximately the same power as GPS signals, and Code Division Multiple Access (CDAA) is used to share the L1 channel. In addition, position determination devices that use the WAAS do not need an additional antenna and receiver since the GPS antenna and receiver are used to pick up the WAAS signal. However, prior art systems are designed either to receive and process WAAS signals (on the existing L1 receiver of the GPS position determination device), or to receive and process RTCM signals (using a radio receiver operating in the 300 kHz range), or to receive and process correction data in a particular manufacturer's format (typically at a frequency in the unlicensed frequency band). Thus, prior art systems that use a particular manufacturer's format are not RTCM compatible. That is, they cannot use RTCM signals for accurately determining position. In addition, systems that are designed to receive and process WAAS signals are not RTCM compatible (they cannot use RTCM signals for accurately determining position).
In operations that use position determination devices, it is often necessary to use data from other sources in conjunction with locations determined using the position determination device. For example, in surveying operations, it is often necessary to use data from a laser range finder in conjunction with locations determined using a position determination device. Laser range finders and other similar devices (external data devices) typically include a standard communication port. This standard communication port can be used to connect the laser range finder to the position determination system via a standard electrical cable. However, the use of an electrical cable inhibits separate and independent use of the position determination system. That is, the position determination system cannot move any further from the laser range finder than the electrical cable will allow. Also, cables often break. Furthermore, the cables are a nuisance, making handling of components connected by cable difficult and clumsy.
What is needed is a position determination system that is easily moved from place to place, that is easy to use, and can use RTCM correction data when it is available and when it is required for accurately determining position. Also, what is needed is a method and apparatus for coupling correction data from RTCM signals to a position determination device. Also, a method and apparatus for coupling data from external data devices to a position determination device is required that does not require the use of cables. The present invention provides a solution to the above needs.